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One of the more useful extensions is the state extension, which can easily detect tracking information for a packet Connection tracking maintains information about a connection such as its source, destination, and port It provides an effective means for determining which packets belong to an established or related connection To use connection tracking, you specify the state module first with -m state Then you can use the --state option Here you can specify any of the following states:

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State NEW ESTABLISHED RELATED INVALID RELATED+REPLY Description A packet that creates a new connection A packet that belongs to an existing connection A packet that is related to, but not part of, an existing connection, such as an ICMP error or a packet establishing an FTP data connection A packet that could not be identified for some reason A packet that is related to an established connection but is not part of one directly

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If you are designing a firewall that is meant to protect your local network from any attempts to penetrate it from an outside network, you may want to restrict packets coming in Simply denying access by all packets is unfeasible because users connected to outside servers say, on the Internet must receive information from them You can, instead, deny access by a particular kind of packet used to initiate a connection The idea is that an attacker must initiate a connection from the outside The headers of these kinds of packets have their SYN bit set on and their FIN and ACK bits empty The state module s NEW state matches on any such SYN packet By specifying a DROP target for such packets, you deny access by any packet that is part of an attempt to make a connection with your system Anyone trying to connect to your system from the outside is unable to do so Users on your local system who have initiated connections with outside hosts can still communicate with them The following example will drop any packets trying to create a new connection on the eth0 interface, though they will be accepted on any other interface:

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You can use the ! operator on the eth0 device combined with an ACCEPT target to compose a rule that will accept any new packets except those on the eth0 device If the eth0 device is the only one that connects to the Internet, this still effectively blocks outside access At the same time, input operation for other devices such as your localhost are free to make new connections This kind of conditional INPUT rule is used to allow access overall with exceptions It usually assumes that a later rule such as a chain policy will drop remaining packets

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iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

Part V:

Security

TIP You can use the iptstate tool to display the current state table

Specialized Connection Tracking: ftp, irc, Amanda, tftp

To track certain kinds of packets, IPtables uses specialized connection tracking modules These are optional modules that you have to have loaded manually To track passive FTP connections, you would have to load the ip_conntrack_ftp module To add NAT table support, you would also load the ip_nat_ftp module For IRC connections, you use ip_conntrack_irc and ip_nat_irc There are corresponding modules for Amanda (the backup server) and TFTP (Trivial FTP) If you are writing your own IPtables script, you would have to add modprobe commands to load the modules

Network address translation (NAT) is the process whereby a system will change the destination or source of packets as they pass through the system A packet will traverse several linked systems on a network before it reaches its final destination Normally, they will simply pass the packet on However, if one of these systems performs a NAT on a packet, it can change the source or destination A packet sent to a particular destination can have its destination address changed To make this work, the system also needs to remember such changes so that the source and destination for any reply packets are altered back to the original addresses of the packet being replied to NAT is often used to provide access to systems that may be connected to the Internet through only one IP address Such is the case with networking features such as IP masquerading, support for multiple servers, and transparent proxying With IP masquerading, NAT operations will change the destination and source of a packet moving through a firewall/gateway linking the Internet to computers on a local network The gateway has a single IP address that the other local computers can use through NAT operations If you have multiple servers but only one IP address, you can use NAT operations to send packets to the alternate servers You can also use NAT operations to have your IP address reference a particular server application such as a web server (transparent proxy) NAT tables are not implemented for ip6tables